Luminescence sensitizing ligands

As it is well known, sensitization of Ln-centered luminescence can be achieved via an intramolecular energy transfer upon excitation of organic ligands, instead of using direct excitation of the weak Lnm absorption bands. This phenomenon now called antenna effect or luminescence sensitization has first been observed in 1942 by Weissman for europium(III) complexes formed with salicylaldehyde and with /3-diketonates, more particularly benzoyl-acetonate (ba, 48a), dibenzoylmethanate (dbm, 48b) and wieta-nitrobenzoylacctonatc (47a, fig. 41) (Weissman, 1942). 

To assess the effect of distance between the antenna chromophore and the Eu(III) center on luminescence sensitization, a series of four additional Eu(III) complexes, of ligands 57-60, each having the pendant phenanthridyl group at a different distance from the Eu(III) center, have been synthesized for comparison with [Eu(46)] + (76). 

Since the dipole strength of/-/ transitions are formally forbidden, typically, these extinction coefficients are of the order of 1 M cm an alternative path has to be used which is called luminescence sensitization or antenna effect, that is when the luminescent ion is coordinated with an organic ligand or imbedded into a matrix, then the energy absorbed will be transferred from the surrounding onto the luminescent ion and subsequently the ion emits characteristic light. 

Energy transfer from the ligand, evidenced for the first time in 1942 by Weissman (1942), is often referred to as the antenna effect (Sabbatini et al., 1996) or luminescence sensitization (Biinzli and Piguet, 2005). The overall quantum yield of the metal luminescence upon ligand excitation can be separated into three components characterizing successively (1) the rate constant =kf Q hiplet states) for the... 

Finally, some illustrative examples of sensitizing ligands are examined. The intent here is to provide, with a few structures, a broad overview of the trends and strategies used to improve or modulate the properties of a luminescent lanthanide complex. The examples are used to point at important considerations when attempting to design a ligand. This chapter is not a review of the whole complexes and apphcations which can be found in literature or patents, but aims at illustrating some of the aspects of the luminescent complexes, with arbitrary chosen examples. The fundamental concepts of photophysics and luminescence explained in Luminescence are a prerequisite to hilly understand the notions presented here. 

Illarionova, V. A., et al. (1997). Removal of essential ligand in N-terminal calcium-binding domain of obelin does not inactivate the photoprotein or reduce its calcium sensitivity, but dramatically alters the kinetics of the luminescent reaction. In Hastings, J. W., et al. (eds.), Biolu-min. Chemilumin., Proc. Int. Symp., 9th, 1996, pp. 431 —434. Wiley, Chichester, UK. 

BCPDA is a tetradentate ligand, and forms 1 1 complexes in these conditions. The metal ion is thus coordinatively unsaturated, and also binds a number of water molecules, which can lead to non-radiative quenching of the metal-centered excited state (vide supra). These must be removed by drying to achieve optimal luminescence intensity. This form of assay is, in fact, slightly less sensitive than DELFIA. 

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